Which versions are supported? Which ones are compatible with which versions?

The current LTS release (0.14.x) is compatible with versions going back to v0.7.8.
However, just because it is possible to connect does not mean that you should be using an outdated version. Outdated versions contain known bugs and vulnerabilities.

Issues - known problems

Try a different encoding (ie: --encoding=png) and use packaged versions that are known to work, if you build from source you may need to apply some patches to match your specific environment

Xpra crashed! Can I recover my session?

Generally yes, as long as the virtual display server (vfb) has not crashed.
If the xpra server is completely gone, you can start a new one with "xpra start --use-display" to re-use the existing display. If it is still running but unresponsive, you should kill it first (and use kill -9 to prevent the tear down code from also stopping the vfb display)

Why does Xpra use so much of my CPU?

sound forwarding will consume a little bit of CPU and bandwidth. Turn if off if you don't need it.
Some applications will also repaint their windows unnecessarily, if you don't use them, try minimizing the window.

As per above, those tools will do their own clipboard synchronization which will definitely cause problems - try disabling one of the clipboard synchronization mechanism, and if possible, do not layer remote desktop protocols on top of each other

I cannot click on menus

If you are using Qt/KDE, this is known bug which has been fixed so you will need up to date Qt/KDE libraries, see the platforms section below

I cannot use Xpra with NFS home

Use the socket-dir option to place Xpra's log files and sockets in a location that is not NFS mounted (ie: /tmp/)

Please see the --mmap-group option: the two users need to be in the same group, the mmap file created by the client will use the group-id of the server socket file. You must generally also use the --socket-dir option to place the socket in a location which is available to both users (ie: /tmp)

How do I theme my application?

That depends on the application. GTK2 applications can be themed by setting the GTK2_RC_FILES environment variable, ie:

Warnings and Messages

"cannot create group socket '/run/xpra/USERNAME'", usually followed by [Errno 13] Permission denied

Harmless warning, safe to ignore. The server tries to create a socket in the shared group directory /run/xpra. This is only useful for sharing access to sessions via unix group membership, in combination with the socket-permissions option.

Failed to rename log file "/run/user/$UID/xpra/Xorg.S$PID.log" to "/run/user/$UID/xpra/Xorg.S$PID.log": No such file or directory

This harmless warning is caused by a bug in your Xorg server, see #1192. Complain to your distribution if this bothers you. This is safe to ignore.

"Xlib: extension "RANDR" missing on display" and "Randr not supported: X server does not support required extension Randr"

The virtual framebuffer (typically Xvfb) does not support Randr, this may cause some display problems, try using Xdummy instead

"loop nesting too deep" and "you may have a clipboard forwarding loop, disabling the clipboard"

another piece of software is trying to synchronize the clipboard contents (maybe a clipboard manager, or a virtualization solution like vmware or virtualbox) and this is conflicting with xpra's own mechanism, so xpra stops trying to fight with it - you should disable the other clipboard synchronization if you want xpra clipboard sharing

"found an existing window manager on screen ..."

Xpra is a window manager, you cannot run two window managers on the same display at the same time. If you want to forward a whole desktop, including its window manager, see Usage: forwarding a whole desktop, otherwise stop the other window manager.

"cannot register our notification forwarder ..."

The xpra server was started from a GUI session which already had a dbus instance and a notification daemon, notifications forwarding will not work.
See "How do I use notifications forwarding" (above)

"re-starting speaker because of overrun"

This usually means that there is too much network jitter and that sound packets are arriving in bursts. You can try a different sound codec (--speaker-codec option) or just turn off sound if you don't need it. More info here: wiki/Sound

"Warning: pulseaudio has terminated. Either fix the pulseaudio command line or use --no-pulseaudio to avoid this warning."

This means that xpra failed to create a window matching the desired constraints, you may experience display artifacts around the window's borders

"dec_avcodec is probably leaking memory"

Your installation should be using dec_avcodec2 (for ffmpeg version 2 onwards) or it is missing some patches

"Xorg is not readable"

Your distribution ships Xorg suid but it is not readable by your current user, since Xpra needs to run Xorg non-suid, it needs to make a non-suid copy of the file so you will have to make it readable (please complain to your distributor - there is nothing we can do about this and the security benefits of preventing read access to a suid binary are dubious at best in most cases).
For more details, see Xdummy.

"{{{DPI set to NN x NN (wanted MM x MM), you may experience scaling problems, such as huge or small fonts, etc - to fix this issue, try the dpi switch, or use a patched Xorg dummy driver"

You need to install the patched dummy driver which is available in the xpra repositories. More information here: Xdummy

This means that the user you login as on the server has never had an xpra server running. You must be using the wrong username or even the wrong server.
Try adding the username to your ssh connection string:

xpra attach ssh:username@host

I get a GPG error: KEYEXPIRED 1273837137

The key had expired. Try re-importing the updated key.
On Debian you may have to delete the key (apt-key -d) before adding it again.

During installation I see warnings like these:

chcon: can't apply partial context to unlabeled file

If you have disabled SELinux you can ignore those warnings, otherwise you may need to relabel your filesystem and ensure that the xpra codecs mentioned in those warnings are labelled correctly to prevent runtime codec failures: chcon -t texrel_shlib_t CODECFILE

Warning: xpra server does not support initenv

Your server is out of date, upgrade it. This may cause remote ssh start to fail.

X11 keyboard warnings:

Unsupported high keycode 372 for name <I372> ignored
X11 cannot support keycodes above 255.
This warning only shows for the first high keycode.

OpenGL Issues

No, it is optional but strongly recommended in order to achieve good performance.

No module named gdkgl

Install ​gtkglext and its Python bindings to enable support for OpenGL.

How do I enable OpenGL or other X11 extensions on the server?

Please see Xdummy, for acceleration see ​virtualgl, usually just need to run vglrun YOURAPP from within the xpra session.

"No OpenGL_accelerate module loaded"

You can install the required module: ​OpenGL_accelerate or you can ignore this warning, it only means that some OpenGL operations will not be fully accelerated. (Note, if you install python-opengl via distro package manager, e.g. apt, you should not install OpenGL_accelerate directly using pip. That would likely install the latest version which may well be incompatible with PyOpenGL installed via package manager. Rather, you should go to OpenGL_accelerate website and download the version that is the same with your python-opengl package and install it manually!)

These warnings can safely be ignored. It means that your python OpenGL installation is incomplete, either install the missing modules (usually python-numeric) or if you are building from source, you can apply the silence-pyopengl-formathandler-warnings.patch patch to silence those warnings.

I get errors like these:

No module named Numeric
No module named numpy

This is the same issue as the one above, same answer.

Why don't you fix those OpenGL warnings/errors? Or silence warnings by default?

Version 0.12 does. Note: this is a packaging issue (see ​redhat bugzilla, ​debian bug), and those are not our packages. Silencing warnings by default is bad practice, some of those messages may well be legitimate. The developers/packagers should remove those that are not.
Importing a library should not show ugly warnings on screen, especially when the missing libraries it complains about are deprecated and not available for installation. You can see this for yourself with:

System Tray

There is no system tray with Ubuntu.

There is. Just don't use the packages shipped by Ubuntu, which are outdated and lack the necessary dependencies.

There is no system tray with gnome shell

Please complain to gnome for breaking things, they removed the system tray, see #476. There are ​extensions you can use to restore the system tray area (​topicons is one). With v0.11 onwards, you can still access the session information window using the Meta+Shift+F11 shortcut.